Compositions and methods for the separation of metals

ABSTRACT

Methods and compositions are described for preparing metal salts and selectively separating metals from substrates and other metals. The methods can include a thionyl reagent that reacts with a metal in a solution to produce a metal salt. The reaction can be controlled by varying reagents and conditions such that the method can be used to selectively separate one or more metals from another metal or from a substrate. The method can also be used for removing metals from a surface. Compositions produced by the method are also described.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application No. 61/379,857 filed 3 Sep. 2010, the entire contents and substance of which are hereby incorporated by reference in its entirety as if fully set forth below.

TECHNICAL FIELD

The various embodiments of the present invention relate generally to the reaction of thionyl reagents with metals, the use of that reaction to selectively separate metals from mixtures and more particularly to compositions and methods for achieving the separation.

BACKGROUND

Dissolution of noble metals is important for metallurgy, catalysis, organometallic chemistry, syntheses and applications of noble metal nanoparticles, and recycling of noble metals. Aqua regia (“royal water”) has been used for centuries as a powerful etchant to dissolve noble metals. Aqua regia is a simple 1:3 mixture of concentrated nitric and hydrochloric acids can dissolve noble metals such as gold, palladium, and platinum while these metals are not soluble in either of the acids independently. However, conventional platinum recovery is complicated by its reliance on aqua regia and possible subsequent precipitation of the dissolved platinum from the solution. Aqua regia dissolves all the metals at the same time and cannot separate them out, resulting in low-purity platinum which cannot go back to the product stream directly; it has to be refined, but refining is costly. Moreover, aqua regia is relatively dangerous to work with, is not readily recyclable, and is considered by many to be environmentally hazardous.

This issue is compounded by the current global energy crisis demands for green energy technologies, which will undoubtedly require increased noble metal resources. However, noble metals are scarce, and the current metal prices, particularly for copper, silver and gold reported in the daily news, reveal a need for new methods of recovering these metals. The ability to recover high-purity noble metals via recycling processes may be paramount to the sustainable development. Among the noble metals, platinum is widely used as a catalyst in many green technologies, in particular, proton-exchange membrane fuel cell.

Another class of materials, bimetallic nanoparticles of noble metal elements, will also require selective separation of metals during recycling. Among the various combinations of bimetallic nanoparticles, Au—Pt core-shell nanoparticles (sometimes called platinum-decorated gold nanoparticles, platinum-layered gold nanoparticles, or gold-platinum dendritic hetero-aggregate nanoparticles to account for the morphology of the non-uniform and porous platinum shell) are an important category that has demonstrated improved catalytic activities and multifunctionality. Platinum is becoming increasingly important because of growth in environmentally-friendly applications such as fuel cells and pollution-control catalysts. Researchers, especially materials chemists, have been trying to push platinum utilization to its theoretical maximum; this accounts significantly for the broad interest in platinum-metal (M=Au, Pd, Ni, etc.) bimetallic nanoparticles. Efficient recovery of platinum elements from catalyst nanoparticles is desirable from the viewpoint of sustainable growth.

BRIEF SUMMARY

The various embodiments of the present invention provide methods and compositions for reactions of metals with thionyl reagents, methods of selectively separating metals from substrates, methods of selectively separating metals from other metals, methods of removing metals from materials, compositions for conducting the methods, and compositions resulting from the methods.

An exemplary embodiment of the present invention is a method for preparing a metal salt. The method comprises reacting a metal with a solution comprising SOX¹X², wherein X¹ is Cl or Br and X² is a halide, OR¹ or NR²R³ and R¹, R², and R³ are each independently a C₁-C₆ alkyl, alkenyl, alkynyl, or carbonyl. In an embodiment, X² can be Cl, Br, or OR. In another embodiment, SOX¹X² can be SOCl₂.

In some exemplary embodiments of the present invention, the metal can be any metal that is not a passivated metal. In an exemplary embodiment, the metal can be copper, nickel, iron, tin, indium, silver, gold or palladium. In other exemplary embodiments, the metal can be silver, gold or palladium.

In some exemplary embodiments of the present invention, the solution can contain a donor solvent. In an exemplary embodiment, the donor solvent can be N,N′-dimethylformamide, pyridine, imidazole, thiazole, oxazole, pyridazine, pyrimidine, pyrazine, triazine, pyrrolidine, pyrrolidone, isoxazole, isothiazole, or pyrazole. In another exemplary embodiment, the donor solvent can be N,N′-dimethylformamide, pyridine or imidazole.

Another exemplary embodiment of the present invention is a method for separating at least one metal from a substrate, the method comprising introducing the substrate by solution phase contact or vapor phase contact to a solution comprising SOX¹X², reacting the at least one metal with at least a portion of the SOX¹X² in the solution to form a reaction solution, wherein the reaction solution contains a metal salt of the at least one metal, and separating the reaction solution containing the metal salt from the substrate. The metal can be copper, nickel, iron, tin, indium, silver, gold or palladium, or a combination thereof.

In some exemplary embodiments, reacting the metal can include stirring, heating the solution, sonicating the solution, or causing the solution to flow across the substrate. In some embodiments, the solution can contain a dissolving solvent. The dissolving solvent can be an organic solvent. In one embodiment, the dissolving solvent can be acetonitrile.

The method can also include the steps of recovering at least a portion of the substrate that was separated from the reaction solution, and repeating the previous steps with the recovered substrate. In some embodiments, the substrate can include platinum.

Another exemplary embodiment of the present invention can be a reaction solution containing a metal salt, a thionyl reagent and a donor solvent. In an embodiment, the thionyl reagent can be SOCl₂ or SOBr₂. The metal salt can be a copper salt, a silver salt, a gold salt, a palladium salt or a combination thereof. In an embodiment, the donor solvent can be pyridine, DMF, or imidazole.

According to another exemplary embodiment, a method for selectively separating at least one metal from a substrate that has platinum and the at least one metal comprises introducing the substrate to an organic solution comprising SOX¹X² and a donor solvent, reacting the at least one other metal with the organic solution to form a reaction solution, wherein the reaction solution contains a metal salt of the at least one other metal, and separating the reaction solution containing the metal salt from the substrate containing the platinum. The X¹ can be Cl or Br and X² can be a halide, OR¹ or NR²R³, where R¹, R², and R³ are each independently a C₁-C₆ alkyl, alkenyl, alkynyl, or carbonyl. The donor solvent can be N,N′-dimethylformamide, pyridine, imidazole, thiazole, oxazole, pyridazine, pyrimidine, pyrazine, triazine, pyrrolidine, pyrrolidone, isoxazole, isothiazole, or pyrazole. In another embodiment, the donor solvent can be N,N′-dimethylformamide, pyridine or imidazole. The at least one metal can be copper, nickel, iron, tin, indium, silver, gold or palladium or a combination thereof. In another embodiment, the at least one metal can be gold, silver, or palladium.

Another exemplary embodiment of the present invention is a method for separating a noble metal from a substrate comprising introducing the substrate to an organic solution having SOX¹X² and a donor solvent, reacting the noble metal with at least a portion of the SOX¹X² in the organic solution to form a reaction solution, where the reaction solution contains a noble metal salt of the noble metal, and separating the reaction solution containing the noble metal salt from the substrate. In an embodiment, X¹ can be Cl or Br and X² can be a halide, OR¹ or NR²R³, and R¹, R², and R³ are each independently a C₁-C₆ alkyl, alkenyl, alkynyl, or carbonyl. In another embodiment, X¹ and X² are each independently Cl or Br. In an embodiment, the noble metal can be gold, silver or palladium. In an embodiment, the donor solvent can be N,N′-dimethylformamide, pyridine, imidazole, thiazole, oxazole, pyridazine, pyrimidine, pyrazine, triazine, pyrrolidine, pyrrolidone, isoxazole, isothiazole, or pyrazole. In another embodiment, the donor solvent is N,N′-dimethylformamide, pyridine or imidazole.

The method can further include steps prior to the step of introducing the substrate to the organic solution having SOX¹X² and a donor solvent. In an embodiment, the substrate can be first treated with a non-donor solvent solution comprising SOX¹X² without a donor solvent;, then the substrate can be separated from the non-donor solution and subjected to the remaining steps of the method.

According to an exemplary embodiment of the present invention, a method for separating a non-noble metal from a substrate comprises introducing the substrate to an organic solution comprising SOX¹X², reacting the non-noble metal in the substrate with the organic solution to form a reaction solution, wherein the reaction solution comprises a salt of the non-noble metal from the non-noble metal on the substrate, and separating the reaction solution containing the copper salt from the substrate. X¹ can be Cl or Br and X² can be a halide, OR¹ or NR²R³, and R¹, R², and R³ are each independently a C₁-C₆ alkyl, alkenyl, alkynyl, or carbonyl. In an embodiment, X¹ and X² can be each independently Cl or Br. In an embodiment, the organic solution contains acetonitrile. In an embodiment, the non-noble metal can be copper.

The method can be used to prepare a copper pattern on a surface. In an exemplary embodiment, the method comprises depositing a layer of copper on a surface, placing a pattern mask over the copper layer, and treating the surface of the device with an organic solution containing SOX¹X² to remove the copper exposed outside the mask.

Other aspects and features of embodiments of the present invention will become apparent to those of ordinary skill in the art, upon reviewing the following detailed description in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates rates of reaction for different metals in accordance with exemplary embodiments of the present invention.

FIG. 2 illustrates a reaction flow chart in accordance with exemplary embodiments of the present invention.

FIG. 3 illustrates a vapor phase etching system in accordance with exemplary embodiments of the present invention.

DETAILED DESCRIPTION

To facilitate an understanding of the principles and features of the various embodiments of the invention, various illustrative embodiments are explained below. Although exemplary embodiments of the invention are explained in detail, it is to be understood that other embodiments are contemplated. Accordingly, it is not intended that the invention is limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways. Also, in describing the exemplary embodiments, specific terminology will be resorted to for the sake of clarity.

It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural references unless the context clearly dictates otherwise. For example, reference to a component is intended also to include composition of a plurality of components. References to a composition containing “a” constituent is intended to include other constituents in addition to the one named.

Also, in describing the exemplary embodiments, terminology will be resorted to for the sake of clarity. It is intended that each term contemplates its broadest meaning as understood by those skilled in the art and includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.

Ranges may be expressed herein as from “about” or “approximately” one particular value and/or to “about” or “approximately” another particular value. When such a range is expressed, other exemplary embodiments include from the one particular value and/or to the other particular value.

By “comprising” or “containing” or “including” is meant that at least the named compound, element, particle, or method step is present in the composition or article or method, but does not exclude the presence of other compounds, materials, particles, method steps, even if the other such compounds, material, particles, method steps have the same function as what is named.

It is also to be understood that the mention of one or more method steps does not preclude the presence of additional method steps or intervening method steps between those steps expressly identified. Similarly, it is also to be understood that the mention of one or more components in a composition does not preclude the presence of additional components other than those expressly identified.

Various embodiments of the present invention are directed to the reaction of thionyl reagents with metals. Treating various metals with a thionyl reagent, such as, but not limited to, thionyl chloride, along with a solvent, such as, but not limited to, pyridine, causes the metal to react and oxidize, creating a metallic solution. Various embodiments of the present invention further comprise a series of reactions that can cause a reaction for metals on a surface and can allow for converting metals to salts, and even can allow for selective conversion in the presence of other metals.

Various embodiments of the present invention are reactions of a metal with SOX¹X², which can be referred to as a thionyl reagent. X¹ and X² can be the same or different, and can be halides, alkoxides, or amides. In an embodiment, the alkoxide, which is chemically related to an alcohol, can be described as —OR¹. In an embodiment, the amide, which is chemically related to an amine, can be described as —NR²R³. R¹, R², and R³ can be each independently a C₁-C₆ alkyl, a C₁-C₆, a C₁-C₆ alkenyl, a C₁-C₆ alkynyl, or a C₁-C₆ carbonyl compound. In an embodiment, X¹ is chloride or bromide. In another embodiment, X¹ and X² can each be independently chloride or bromide. SOX¹X² can be SOCl₂, commonly referred to as thionyl chloride, or can be SOBr₂, commonly referred to as thionyl bromide.

In an exemplary embodiment of the present invention, a metal reacts with the thionyl reagent. The metal can be a metal in a zero oxidation state in which the reaction of the zero oxidation state metal can be an oxidation to a positive oxidation state by the oxidation with the thionyl reagent. In an embodiment, the metal can be any metal known by one of ordinary skill in the art to react with a thionyl reagent. The metal can be nickel, copper, iron, tin, or indium, referred to herein as non-noble metals. The metal can be gold, silver or palladium, referred to herein as noble metals. Metals that do not react in the current method can include platinum, tungsten, chromium, titanium, tantalum, and silicon.

Without being bound by theory, part of the reactivity for the metals seems to depend on the passivation of the surface of the metal. Metals that have a passivated surface may not react in the current system. For example, titanium is well-known for the strength and hardness of their surface oxides, but does not react in the current system. Platinum is also known to have a hard passivation layer, and is similarly resistant to the reactions of the present invention. The passivated metal need not be actively passivated, such as when titanium is anodized, but can include naturally passivated metals. Therefore, in another embodiment of the present invention, the metal can be any metal that is not a passivated metal.

In various embodiments of the present invention, the reaction of a metal with a thionyl reagent can produce a metal cation and can also produce a counterion, which will be anionic. The metal cation and anionic counterion can form a metal salt. The overall charge of the metal salt, comprising the metal cation and anionic counterion, can be in any overall charge state, including +3, +2, +1, neutral, −1, −2, −3, and so forth. The metal salt can be in any physical state known to one of ordinary skill in the art. The metal salt can be in solution, suspension, precipitated, and so forth. The anion of the metal salt can be any anion recognized by one of ordinary skill in the art. In an embodiment, the anion can be a chloride, bromide, alkoxide, or amide salt. In another embodiment, the anion can be bromide or chloride. The anion can also be chloride.

In the present invention, a solution that can contain the thionyl reagent which reacts with a metal can also contain a solvent. In an embodiment, the solvent is an organic solvent. When the solvent is an organic solvent, the solution can also be described as an organic solution. The organic solvent can be a donor solvent or a dissolving solvent, or both a donor solvent and a dissolving solvent.

In an embodiment, the donor solvent can be any organic solvent that activates the thionyl reagent, reduces the work function of the metal being reacted, or does both. In an embodiment, the donor solvent can be N,N′-dimethylformamide, pyridine, imidazole, thiazole, oxazole, pyridazine, pyrimidine, pyrazine, triazine, pyrrolidine, pyrrolidone, isoxazole, isothiazole, pyrazole, indole, quinoline, purine, pteridine, phthalocyanine, N,N′-dicyclohexylcarbodiimide, N,N′-dimethylbenzylamine, dodecyltrimethylammonium bromide, tri-p-tolyl-phosphine, pyrrole, pyrazolone, or bipyrazole. In another embodiment, the donor solvent can be N,N′-dimethylformamide, pyridine, imidazole, thiazole, oxazole, pyridazine, pyrimidine, pyrazine, triazine, pyrrolidine, pyrrolidone, isoxazole, isothiazole, or pyrazole. In yet another embodiment the donor solvent can be N,N′-dimethylformamide, pyridine, or imidazole.

Without being bound by theory, the donor solvent appears to play two roles in the reaction encompassed by the method of the present invention. First, the donor solvent can coordinate to the surface of the metal, potentially via non-bonding electron pairs or π-bond type coordination. This would effectively reduce the work function of the metal making the surface more susceptible to oxidation. Second, the donor solvent may form a charge-transfer complex with the thionyl reagent to activate it. This might potentially stabilize the complex for the dissociation of the chloride loss and for accepting the electrons from the oxidized metal. This proposed mechanism then allows for tuning the reactivity of the solution to different metals, as embodied by the method in several ways discussed herein.

The reaction rate for the method of the present invention can be controlled by adjusting the ratio of thionyl reagent and donor solvent. In general, the reaction rate varies for the ratio of thionyl reagent:donor solvent based on two competing factors. First, as discussed above, the activation of the complex and the work function of the metal are both affected by increasing donor solvent. Second, the ability of the metal chloride to dissolve away from the substrate is affected by solvent polarity which is dominated by the high-polarity thionyl reagent. By way of a nonlimiting example, the dissolution/reaction rates for a gold metal in a 6:4 thionyl reagent: donor solvent solution can be faster than in a 4:6 solution. Similarly, a 7:3 thionyl reagent: donor solvent can be faster than both a 9:1 solution and a 5:5 solution.

In an embodiment, the ratio of thionyl reagent:donor solvent can be in the range of about 10,000:1 to about 1:10,000. In an embodiment, the ratio can be in the range of about 100:1 to about 1:100, including in the range of about 10:1 to about 1:10. In an embodiment, the ratio can be in the range of about 7:1 to about 1:7, including in the range of about 5:1 to 1:5. In an embodiment, the ratio can be about 3:1.

The rate can also be controlled by adjusting the donor solvent used in the reaction. For example, pyridine, DMF and imidazole give the swiftest reactions for the method with a metal, for example, gold. Other donor solvents can be relatively slower. In a qualitative sense, the “fast” donor solvents can be N,N′-dimethylformamide, pyridine, imidazole; the “medium” donor solvents can be thiazole, oxazole, pyridazine, pyrimidine, pyrazine, triazine, pyrrolidine, pyrrolidone, isoxazole, isothiazole, or pyrazole; and the “slow” donor solvents can be indole, quinoline, purine, pteridine, phthalocyanine, N,N′-dicyclohexylcarbodiimide, N,N′-dimethylbenzylamine, dodecyltrimethylammonium bromide, tri-p-tolyl-phosphine, pyrrole, pyrazolone, or bipyrazole. The slowest solvents, many to the point of not demonstrating any substantial oxidation with some metals, can be acetonitrile, maleimide, azobisisobutyronitrile, aniline, polyaniline, phenanthroline, methylbenzyl cyanide, 2-acetyl-1-methylpyrrole, or benzyltriethylammonium tetrafluoroborate.

The reaction rate for the method of the present invention can also be controlled by dilution of the solution containing the thionyl reagent. Diluting the solution with, for example, an organic solvent, can lead to a slower reaction. However, the diluted reaction can also be more efficient because it cuts down on side reactions, e.g. oligomerization of donor solvent, that consume the thionyl reagent.

The reaction rate of the thionyl reagent with the metals in the method of the present invention can also vary based on the metal being converted to a metal salt. Across several donor solvents, the following relative order of reactivity can be established: silver can be faster than gold and both silver and gold can be faster than palladium. Platinum does not react in the current method. Copper, nickel, iron, tin and indium can be all faster than silver or gold. Silicon, silicon dioxide, titanium, tungsten, tantalum, and chromium do not react in the current method. Teflon is also inert.

In an exemplary reaction according to various embodiments of the present invention, some non-noble metals can react according to the methods of the invention with thionyl reagents and a very weak donor solvent or dissolving solvent. In an embodiment of the present invention, a dissolving solvent can be any organic solvent that solubilizes and stabilizes the metal and/or metal salt to aid the reaction. In one exemplary embodiment, the dissolving solvent can be acetonitrile. Without being bound by theory, some donor solvents do not support the reaction of the thionyl reagents with noble metals, e.g. acetonitrile, but can nonetheless activate the oxidation and dissolution of non-noble metals and thereby act as dissolving solvents. Without being bound by theory, this could be due to the smaller work function present in non-noble metals, e.g. the smaller work function for copper versus gold. Therefore, in some embodiments, the dissolving solvent does not have to be as strong an actor as the donor solvent.

In an embodiment of the present invention, the method can be a reaction of a non-noble metal with a thionyl reagent and a dissolving solvent. In an embodiment, the non-noble metal can be copper, nickel, iron, tin or indium. In another embodiment, the metal can be copper.

In an embodiment of the methods of the present invention, the reaction of the metal with the solution containing a thionyl reagent can include any technique for controlling the reaction that is known to one of ordinary skill in the art. The reaction can include stirring the solution containing the thionyl reagent and the metal, heating the solution, or refluxing the solution. The reaction can be conducted at room temperature, above room temperature, or below room temperature. The reaction can include causing the solution to flow across a surface containing the substrate. The reaction can be conducted in a solution phase, where the substrate and/or metal is immersed or rinsed with the solution containing the thionyl reagent. The reaction can be conducted in the vapor phase, where the substrate is immersed in a vapor containing the solution containing the thionyl chloride. The reaction can be conducted with sonication.

In an embodiment of the present invention, a method can include a step of introducing a substrate to a solution. The solution can also be called a first solution, a reagent solution, a thionyl solution, or a reactant solution. The method can also include a step of introducing a substrate to an organic solution including a thionyl reagent, in which the solution can have an organic solvent present in it. The method can also include reacting a metal with the solution to form a reaction solution, wherein the reacted products are present in the reaction solution.

In an embodiment of the present invention, the substrate used in the method can be any material that contains a metal susceptible to removal by the present invention. The substrate can be any material, catalyst, surface, object, structure, residue, and so forth that has a metal. Non-limiting examples of substrates can include, but are not limited to, automobile combustion catalysts and catalytic converters, petroleum cracking catalysts, industrial catalysts on supports, industrial catalysts in particulate forms, residual catalyst sludges from catalytic reactions, semiconductor and chip manufacturing materials and parts, batteries and anodic or cathodic metals in batteries, metal nanoparticles, fuels cells, crude mining ores and so forth. Some common metal combinations might include, but are not limited to, platinum-nickel, platinum-copper, platinum-palladium, platinum-gold, platinum-iron, platinum-tin, platinum-nickel-gold, platinum-gold-palladium, titania-supported metal compounds, chromium mixed metals, tungsten mixed metals, silicon-metal systems prevalent in semiconductor devices, SiO₂-supported metals systems, metals intermixed with a rock such as granite from mining operation, and so forth.

In view of the above information, methods according to various embodiments of the present invention can include numerous different strategies for dissolving and separating several different metals. In an embodiment, the method can include a solution of thionyl reagent that is fast reacting with any metal, for example, a 3:1 SOCl₂:pyridine solution. Such a reaction could remove all non-passivated metals from a substrate. This technique might be useful for recovering a single metal from a substrate, or for removing a metal from a substrate that contains a passivated metal, e.g. platinum. In an embodiment, the method can include a solution of thionyl reagent that is fast reacting with only non-noble metals, for example a SOCl₂ solution in acetonitrile. Such a solution could remove all non-noble metals, and might be useful for recovering single non-noble metals, or for removing non-noble metals from a substrate containing noble metals. In an embodiment, the method can be used to separate two noble metals, where a first noble metal does not react as swiftly as a second noble metal, e.g. for example, separating silver from palladium. In an embodiment, the method can include a first reaction having SOCl₂ and acetonitrile to remove a non-noble metal, then a second reaction using SOCl₂ and pyridine to remove a noble metal, for example separating copper and gold sequentially from a substrate, or separating copper and gold sequentially from platinum.

Metal salts that can be a product of one or more methods according to various embodiments of the present invention and can be contained in the reaction solution can be recovered by any technique known to one of ordinary skill in the art. In one non-limiting example, the solution containing the metal salt can be evaporated to recover the solids. The solids can then be calcinated to recover the metal. The substrate used in the method can also be further treated by any technique known to one of ordinary skill in the art. In one non-limiting example, platinum on Al₂O₃ can be recovered by dissolving the alumina in a basic solution to recover the metal. In another non-limiting example, platinum could be treated using standard techniques such as aqua regia to recover the purified platinum that would not have other metals present by virtue of the separation achieved with the method embodied in the present invention.

The solutions embodied by the present invention have several advantages over the traditional aqua regia. The solutions embodied by the present invention can be composed of numerous different compositions, and the concentration of those solutions can be varied. This tunability provides for the selective dissolution of metals. In contrast, aqua regia is always 1:3 nitric acid to hydrochloride acid, is a concentrated reaction, and is indiscriminate. Aqua regia is corrosive and explosive, whereas the present solutions are merely corrosive, providing an added safety benefit. Aqua regia is consumed and not recyclable, whereas portions of the solutions embodied by the present invention can be recovered and recycled.

The various embodiments of the present invention are further illustrated by the following non-limiting examples.

EXAMPLES Example 1

Layers of 250-nm-thick Au, Pd, Ag, and Pt, was deposited, onto a 9-cm² silicon substrate each, with 20-nm-thick chromium as the adhesion layer (chromium was not soluble in any reaction medium) to avoid the error that might be introduced due to peel-off of the noble metal metallization layer. The metalized substrates were immersed in 20 mL of 3:1 v/v SOCl₂-py at room temperature with mild shaking for a preset duration, taken out, rinsed thoroughly, dried, and weighed. No weight loss was detected of a platinum film after it was immersed in the SOCl₂-py mixture at room temperature or even at 70° C. (refluxing) for 1 week.

FIG. 1 shows the kinetic results of the dissolution of noble metals in the mixture of SOCl₂ and pyridine (py) with a volumetric ratio of 3:1. SOCl₂ dissolved Au at a rate of 0.3 mol m⁻² h⁻¹ at room temperature, which is faster than Au dissolution in conventional cyanide leaching agents (<0.004 mol m⁻² h⁻¹) and iodide solutions (<0.16 mol m⁻² h⁻¹). Silver (Ag) and Pd also dissolved at high dissolution rates (0.8 mol m⁻² h⁻¹ and 0.5 mol m⁻² h⁻¹, respectively); in comparison, Pt was completely inert.

Example 2

Different solutions in the method were analyzed by different techniques. X-ray photoelectron spectroscopy of the reaction of gold in SOCl₂-py showed the Au 4f^(7/2) binding energy at 86.6 eV characteristic of the oxidation state of Au(III). To demonstrate that Au was oxidized to Au(III) instantaneously, SOCl₂-pyridine vapor was used to etch an Au/Si surface. Au(III) was confirmed on the vapor-etched Au/Si surface by XPS and Raman spectroscopy in the form of AuCl₄ ⁻. Vibration modes of Au(III)-Cl, however, were not observed in the Raman spectrum probably because they were covered by the strong fluorescence background. Actually, after the 0.03 mol L⁻¹ Au/3:1 SOCl₂-py solution was stored at room temperature in a dark room for 150 days, a large amount of yellow precipitate was observed and the solution turned into a dark suspension. [AuCl₄]⁻ was confirmed in the Raman spectrum of the precipitate. Therefore, [AuCl₄]⁻ appeared to form as a result of Au dissolution in SOCl₂-py mixtures. No chemical shifts resulting from coordination of pyridine with Au(III) were observed in the NMR spectra and no simple Au(III) compounds were observed in the mass spectra or the X-ray diffraction pattern. These results suggested the absence of AuCl₃ or its complex with pyridine in the Au—SOCl₂-pyridine solution.

Au(I)→Au(III) oxidation by SOCl₂ has been demonstrated at certain conditions. However, Au cannot be oxidized by SOCl₂ alone. No weight loss was detected for a gold film after immersion in SOCl₂ at 70° C. for 1 week. Pyridine undoubtedly played an important role in the dissolution process. Nuclear magnetic resonance (NMR) spectra of the SOCl₂-py mixture showed a deshielding (low-field chemical shifts) of ¹⁴N (ca, 5 ppm), ¹³C (ca, 1.35 and 2.42 ppm for C-3 and C-4, respectively), and ¹H (ca, 0.35, 0.49 and 0.50 ppm for H-2, H-3 and H-4, respectively), and shielding (high-field chemical shift) of C-2 (ca, -1.77 ppm) relative to pure pyridine. These chemical shifts can be attributed to the charge-transfer interaction between SOCl₂ and pyridine. Charge-transfer complexes between SOCl₂ and amine have been prepared and studied before. In this case, similarly, the sulfur in SOCl₂ is an electron acceptor, and the nitrogen in pyridine is an electron donor. The charge transfer in general weakens the bonds within the acceptor molecule, which accounts for the observed redshifts of both the asymmetric and the symmetric Cl—S—Cl stretching in the Raman spectra of SOCl₂-py mixtures. In comparison with pure pyridine and SOCl₂, shifts and intensity changes of py-related vibration peaks were also observed in the FTIR of the SOCl₂-py mixture. Such shifts are mainly attributed to the formation of the charge-transfer complex, SOCl₂-py, in good agreement with simulation results of the vibration spectra (IR). SOCl₂-py was observed directly in the mass spectrum of the SOCl₂-py mixture. Therefore, the charge-transfer interaction may activate SOCl₂ to oxidize Au.

In the Raman and the FTIR spectra of the SOCl₂-py mixture, almost all the vibration modes of pure pyridine and pure SOCl₂ remained. This indicated that the parent molecules, although perturbed by the charge-transfer interaction, had maintained their basic structural integrity, and that the dominating product was a molecular adduct rather than a rearrangement, elimination or dissociated product. Neither 1-(chlorosulfinyl)-pyridinium chloride nor 1-(chlorosulfinyl)-4-chloro-4-hydropyridine were identified in the NMR spectra. Simulation results showed that 1-(chlorosulfinyl)-pyridinium chloride, the dissociated form of the adduct, was not energy favorable. Proof of its existence could not be picked up easily in the FTIR spectra. However, experimental data of electrical conductivity of the mixtures suggested the presence of mobile ions due to the dissociation. Therefore, 1-(chlorosulfinyl)-pyridinium chloride, though being a weak electrolyte in SOCl₂, existed as the dissociated form of the SOCl₂-py adduct in the solutions. No known reduction products of SOCl₂ such as sulfur have been confirmed.

With the dissolution of Au into the SOCl₂-py mixture, SOCl₂ was consumed non-stoichiometrically. This was evident from the reduced Raman intensities of the vibration peaks at Δv=200.0, 288.5, 348.2, 427.5, and 493.0 cm⁻¹. The py-related peaks were at Δv=657.3, 1000.9, 1035.4, and 3064.6 cm⁻¹ shift to 643.5, 1013.5, 1027.3, and 3101.4 cm⁻¹, respectively. The peak shifting were complete at Au(III) concentration of as low as 0.03 mol L⁻¹. Correspondingly, the NMR spectra of the 0.03 mol L⁻¹ Au—SOCl₂-py solution showed complete chemical shifts of ¹⁴N (ca, −10.4 ppm), ¹³C (ca, −0.46, 0.14, and 0.50 ppm for C-2, C-3, and C-4, respectively) and ¹H (ca, −0.06, −0.04, and −0.04 ppm for H-2, H-3 and, H-4, respectively) relative to the 3:1 SOCl₂-py mixture before Au dissolution. These results suggested a non-stoichiometric chemical reaction in the solution. Solid state ¹³C NMR spectrum showed chemical shifts of C-4, C-2, and C-3 at 147.7, 142.2, and 129.2 ppm, respectively. This was in very good agreement with the calculated ¹³C NMR result of oligomers of 4-chloropyridine. Protonation at the pyridinium site were confirmed by the appearance of the vibration peak of NH⁺ at ˜3222 cm⁻¹ in the FTIR spectra and the Au—SOCl₂-py solution. The pyridinium structure was also reflected in the cyclic voltammogram of the gold/SOCl₂/pyridine solution in acetonitrile, where the reduction peak potential at −1.28 V at the cathodic scan fell in the regime of the reduction potentials of pyridiniums. Dimers and trimers of 4-chloropyridine and their derivatives were present in the mass spectra and the precipitate from the long-standing Au—SOCl₂-py solution. The 4-chloropyridine might have formed as the intermediate and Au(III) catalyzed the oligomerization of 4-chloropyridine. The oligomeric nature was evident by the very viscous solution. The increased viscosity resulted in low mobility of molecules. This low mobility of molecules could be the reason for the peak broadening observed in the ¹³C NMR spectrum of the Au—SOCl₂-py solution (Table S1). Different metals such as Au, Ag, and Cu can exhibit different catalytic effects. The oligomerization reaction accounts for the non-stoichiometric feature.

Example 3

Gold nanoparticles were synthesized by reducing 12 mg HAuCl₄.3H₂O in 10 mL oleylamine at 105° C. for 2 h under an Ar blanket, collected by precipitation with ethanol and centrifugation, and washed with mixtures of acetone and hexane. Platinum nanoparticles were synthesized by reducing 30 mg H₂PtCl₆.6H₂O in 20 mL ethylene glycol (containing 35 mg polyvinylpyrrolidone, M_(w)=29000) at 165° C. for 2 h under an Ar blanket, collected by precipitation with acetone and centrifugation, and washed with mixtures of acetone and ethanol [17].

20 mg dried gold nanoparticles and 5 mg dried platinum nanoparticles were mixed, and then added into 10 mL SOCl₂ to form a dark brown mixture. A certain amount of pyridine was added at room temperature, changing the colour of the mixture to light brown. The solution was diluted into 200 mL acetonitrile. The dissolved Au went into the solution; the non-dissolved nanoparticles were precipitated out of the solution, washed, and collected. The yield was calculated by dividing the weight of the collected recovered platinum nanoparticles by the starting weight of the platinum nanoparticles in the mixture, i.e., 5 mg.

Au—Pt core-shell nanoparticles were synthesized by a sequential reduction reaction in oleylamine. Au core was formed by reducing 24 mg HAuCl₄.3H₂O in 10 mL oleylamine at 120° C. for 3 h under an Ar blanket. 25 mg platinum acetylacetonate was dissolved in 10 ml oleylamine at 70° C., and then added into the dark purple Au colloidal solution at 120° C. under vigorous stiffing. The solution was fast heated up to 235° C., and kept for 3 h. The whole synthesis process was under Ar protection. The core-shell nanoparticles were collected by precipitation with ethanol and centrifugation, and washed with mixtures of acetone and hexane.

10 mg of the dark brown Au—Pt core-shell Nanoparticles were added into a solution of SOCl₂ and pyridine (3:1 in volume) at room temperature, and then diluted into 400 mL acetonitrile. The acetonitrile solution was then mixed with 50 mL hexane under vigorous stiffing. After the mixing, the mixture went through a fast phase separation at room temperature. The dissolved Au went into the bottom acetonitrile phase (yellow). Platinum nanoparticles were extracted out of the acetonitrile solution, and re-dispersed in the upper hexane phase (brown). It is surprising that the oleylamine capping agent is so strongly coordinated to the surface of the Platinum nanoparticles that it results in the easy extraction of the capped platinum nanoparticles by hexane. After the acetonitrile was recycled by distillation and condensation (>95% of the acetonitrile was recovered). Au was recovered simply by calcination. The platinum nanoparticles/hexane dispersion was washed several times by mixing with deionized water to remove the ionic residuals. The platinum nanoparticles were recovered after distillation of the hexane.

XRD patterns of the synthesized Au and platinum nanoparticles were taken. The average sizes of the Au and platinum nanoparticles are approximately 4.8 and 4.3 nm, respectively, based on calculation according to Scherrer's equation (using {111} peaks only, so are the calculations from here after).

The leaching process of the nanoparticle mixtures was monitored by XRD characterizations of the collected precipitate powders: with leaching of the gold nanoparticles from the mixture, the relative intensity of the Pt {111} peak with regard to the Au {111} peak increased. Platinum nanoparticles were recovered after a thorough leaching of the gold nanoparticles. No distinct broadening of the Pt {111} peak after the leaching process was observed, which indicated that the recovered platinum nanoparticles remain intact during the leaching. The purity of the recovered Pt was 99.49±0.22%, determined by ICP-MS. The yield of the recovered Pt was 80-86% (3.9-4.3 out of the 5 mg).

UV-vis spectrum and XRD pattern of the synthesized Au—Pt core-shell nanoparticles were consistent with those reported in literature about Au—Pt core-shell nanoparticles. The average diameter of the Au core (by fitting the XRD pattern and calculation based on Scherrer's equation) is 5.8 nm. A very broad Pt {111} peak was evident, corresponding to 3.2 nm in size. The poor signal-to-noise ratio, the weak {200} and {220} peaks, and the broad {111} peak (corresponding to an average size of 3.2 nm) of the recovered platinum nanoparticles were all characteristic of the poor crystallinity and small size of the platinum nanoparticles initially in the form of shells in the Au—Pt core-shell nanoparticles. UV-vis spectra showed a weak surface plasmon resonance (SPR) peaked at around 510 nm for the core-shell nanoparticles; in contrast, SPR disappeared after the gold nanoparticles were leached thoroughly.

The purity of the recovered platinum is 95.02±0.08%. The purity was relatively low, in comparison with the recovered platinum from the mixture of platinum and gold nanoparticles. A possible reason is that a small amount of gold atoms are mixed indistinguishably into the lattice of the Pt shell during the sequential synthesis process of the Au—Pt core-shell nanoparticles. The recovered platinum and gold were around 2.1 and 6.7 mg, respectively.

Example 4

The thionyl reagent and donor solvent can be formulated using organic solvent/reagents other than pyridine. Many organic solvents/reagents such as N,N-dimethylformamide (DMF), imidazole, pyrimidine, and pyrazine can be used as donor solvents. For example, a SOCl₂-DMF mixture dissolved Au at a rate of 0.3 mol m⁻² h⁻¹; by contrast, neither Pd nor Pt apparently dissolved. One key feature that all the organic candidates have in common is that they can have charge-transfer interactions with SOCl₂. Although quantitative analysis is currently unavailable to answer the question of why different donor solvents show different reactivity toward different noble metals, it can be reasonably assumed that the dissolution selectivity derives from the tunable interaction between SOCl₂ and the donor solvent.

Qualitative relative rates of reaction were determined for a range of donor solvents. Gold particles were suspended in acetonitrile and treated with a solution of SOCl₂ and donor solvent in a 3:1 v/v ratio of SOCl₂:donor solvent. Based on the approximate rate of dissolution, donor solvents were categorized as fast, medium, slow and unreactive. The results are set forth in Table I.

TABLE I FAST MEDIUM SLOW UNREACTIVE pyridine thiazole indole acetonitrile N,N′- oxazole quinoline maleimide dimethylformamide pyridazine purine azobisisobutyronitrile (DMF) pyrimidine pteridine aniline imidazole pyrazine phthalocyanine polyaniline triazine N,N′- phenanthroline pyrrolidine dicyclohexylcarbodiimide methylbenzyl cyanide pyrrolidone N,N′-dimethylbenzylamine 2-acetyl-1-methylpyrrole isoxazole dodecyltrimethylammonium benzyltriethylammonium isothiazole bromide tetrafluoroborate pyrazole tri-p-tolyl-phosphine pyrrole pyrazolone bipyrazole

Example 5

A pure Cu plate (1×1×0.1 cm³, was rough polished with SiC paper (grit 1200 and 2000 sequentially), and fine polished with a Al₂O₃ (50 nm) slurry. The polished Cu plate was cleaned in a 1:1 mixture of ethanol and acetone with sonication, and dried with nitrogen flow. The plate was then fixed in a clamp made of Teflon, and immersed in 200 mL etchant under stirring. The along-thickness direction was parallel to the direction of rotation of the liquid. After a preset duration, the plate was rinsed with distilled water immediately and thoroughly. The plate was dried with nitrogen flow and then weighed. The etching rate, in mg min⁻¹ cm⁻², is calculated from the slope of the line of best fit for the plot of normalized weight loss vs. accumulated immersion time.

TABLE 2 Stirring Etching rate Concentration Temperature rate (mg min⁻¹ Etchant (mol L⁻¹) (° C.) (rpm) cm⁻²) FeCl₃ 3.76 50 N/A 3.5 CuCl₂ 2.33 50 N/A 1.7 FeCl₃ 2 50 1500 27 FeCl₃ 2 30 1500 17 O₂/NH₃•H₂O 0.9 atm/0.74 26 660 0.03 Alkaline N/A 30-60 N/A 13-27 CuCl₂ >2 50-54 N/A 11-22 FeCl₃ >2 43-49 N/A 11-22 SOCl₂/CH₃CN 1 20 500 36 SOCl₂/CH₃CN 2 20 500 53 SOCl₂/CH₃CN 1 20 Sonication 320 FeCl₃ ^(b) 2 50 500 19 FeCl₃ ^(b) 2 20 Sonication 33

The etching rate of Cu in 1 mol L⁻¹ SOCl₂/CH₃CN solution at mild conditions is 36 mg min⁻¹ cm⁻². The rate was much faster than those in the current major etchants such as acidic solutions of FeCl₃ (11-27 mg min⁻¹ cm⁻²) and CuCl₂ (11-22 mg min⁻¹ cm⁻²), and ammoniacal alkaline solutions (13-27 mg min⁻¹ cm⁻²) of higher concentrations and stirring rates at elevated temperatures. The etching rate increased with the concentration of SOCl₂. There are two regimes: the low-concentration one (<0.3 mol L⁻¹) with relatively strong concentration dependence, and the high-concentration one with relatively weak concentration dependence. This was consistent with the observed phenomenon that a porous black matter accumulated on the surface of Cu at high SOCl₂ concentrations. The black matter was Cu₂S formed by chemical reaction between Cu and SOCl₂. Cu₂S reacted with SOCl₂ to become CuCl₂ that is soluble in CH₃CN, but with a lower rate than the reaction between Cu and SOCl₂. Therefore, at high SOCl₂ concentrations, the Cu₂S formed very fast, and accumulated on the Cu surface as a diffusion barrier layer. The effect of diffusion was distinct for etchants of high SOCl₂ concentrations, as can be seen in FIG. 3. Sonication (210 W) removed the Cu₂S from the Cu surface very efficiently, and therefore, improved the etching rate dramatically (320 mg min⁻¹ cm⁻²).

There are at least four advantages for the SOCl₂/CH₃CN etchant. First, the high etching rate provides a method for rapid dissolution or etching of copper. Second, the reaction is the insensitivity to surface oxide on the copper. When the copper surface is initially oxidized Cu, there was no distinct change in the etching rate. Third, it is compatible with current Cu/low-K technology because SiO₂ is not soluble in the etchant. In comparison, SiO₂ can be etched by alkaline etchants. Finally, the etchant can be recovered/regenerated simply by distillation; in comparison.

Example 6

Based on kinetic studies, several efficient processes of selective dissolution of metals are demonstrated. The dissolution selectivity can open up a new avenue toward recycling of noble metals, in particular, platinum. Besides the application in noble metal recovery, the method can find wide applications several industries, including, for example, in microelectronics and mining industries.

A silicon substrate was metalized with a Pd—Au—Pt layer (250-nm thick each by an e-beam evaporation or DC sputtering, with chromium as the adhesion layer) and the Au and Pd were dissolved, sequentially and respectively in the SOCl₂-DMF and the SOCl₂-py mixtures. The process is demonstrated in FIG. 2.

A copper-gold-platinum metal system on a silica support is treated first with SOCl₂ in acetonitrile to separate the copper as a copper chloride, then second with SOCl₂ in pyridine to separate the gold as a gold chloride. The remaining metal was platinum metal on a silica support that can be recovered or reprocessed.

A copper-nickel-platinum particle is treated with a thionyl chloride solution containing actenoitrile to separate the copper and nickel as chloride salts, and the remaining solid contains the platinum metal.

A crude mixture of mining ore containing a mixture of copper and gold is treated with a thionyl chloride solution containing acetonitrile to separate the copper ore as copper chloride, then treated with a thionyl-DMF solution to separate the gold ore as gold chloride.

A mixture of used palladium catalysts and platinum catalysts supported on an activated carbon that can be recovered from an industrial process for recycling is first fired to burn off the carbon support, then the remaining material is treated with a thionyl chloride-pyridine solution to separate the palladium as palladium chloride and recover the platinum as a single metal.

Example 7

Vapor etching of Au metallization on a circuit board by vaporized SOCl₂ and donor solvent was conducted, as shown in FIG. 3. A circuit board having a metalized surface of gold, nickel and copper, part of which was protected with an etching mask, was mounted above a solution of thionyl chloride and pyridine was added to the solution via a syringe feed. A nitrogen purge produced a vapor solution that etched the mounted circuit board. Photos of the circuit board are shown, prior to etching, at 5 minutes and at 15 minutes.

The embodiments of the present invention are not limited to the particular formulations, process steps, and materials disclosed herein as such formulations, process steps, and materials may vary somewhat. Moreover, the terminology employed herein is used for the purpose of describing exemplary embodiments only and the terminology is not intended to be limiting since the scope of the various embodiments of the present invention will be limited only by the appended claims and equivalents thereof.

Therefore, while embodiments of this disclosure have been described in detail with particular reference to exemplary embodiments, those skilled in the art will understand that variations and modifications can be effected within the scope of the disclosure as defined in the appended claims. Accordingly, the scope of the various embodiments of the present invention should not be limited to the above-discussed embodiments, and should only be defined by the following claims and all equivalents. 

1-8. (canceled)
 9. A method for separating at least one metal from a substrate, the method comprising: a) introducing the substrate by solution phase contact or vapor phase contact to a solution comprising SOX¹X²; b) reacting the at least one metal with at least a portion of the SOX¹X² in the solution to form a reaction solution, wherein the reaction solution comprises a metal salt of the at least one metal, and c) separating the reaction solution containing the metal salt from the substrate wherein X¹ is Cl or Br and X² is a halide, OR¹ or NR²R³ and R¹, R², and R³ are each independently a C₁-C₆ alkyl, alkenyl, alkynyl, or carbonyl.
 10. The method of claim 9, wherein X² is Cl, Br, or OR¹.
 11. The method of claim 9, wherein SOX¹X² is SOCl₂.
 12. The method of claim 9 wherein the solution further comprises a donor solvent.
 13. The method of claim 12, wherein the donor solvent is N,N′-dimethylformamide, pyridine, imidazole, thiazole, oxazole, pyridazine, pyrimidine, pyrazine, triazine, pyrrolidine, pyrrolidone, isoxazole, isothiazole, or pyrazole.
 14. The method of claim 12, wherein the donor solvent is N,N′-dimethylformamide, pyridine or imidazole.
 15. The method of claim 9, wherein the solution further comprises a dissolving solvent.
 16. The method of claim 15, wherein the dissolving solvent is an organic solvent.
 17. The method of claim 15, wherein the dissolving solvent is acetonitrile.
 18. The method of claim 9, further comprising d) recovering at least a portion of the substrate that was separated from the reaction solution, and e) repeating steps a through c with the recovered substrate.
 19. The method of claim 9, wherein the at least one metal is copper, nickel, iron, tin, indium, silver, gold or palladium. 20-21. (canceled)
 22. The method of claim 9, wherein the at least one metal is gold, silver or palladium.
 23. The method of claim 9, wherein the at least one metal is gold, silver or palladium, and the solution further comprises a donor solvent.
 24. The method of claim 9, wherein the reacting the metal with the SOX¹X² solution comprises stirring, mixing, controlling the temperature or sonicating the solution. 25-31. (canceled)
 32. A method for selectively separating at least one metal from a substrate, wherein the substrate comprises platinum and at least one other metal, the method comprising the steps of: a) introducing the substrate to an organic solution comprising SOX¹X² and a donor solvent, b) reacting the at least one other metal with the organic solution to form a reaction solution, wherein the reaction solution comprises a metal salt of the at least one other metal, and c) separating the reaction solution containing the metal salt from the substrate containing the platinum, wherein X¹ is Cl or Br and X² can be a halide, OR¹ or NR²R³, R¹, R², and R³ are each independently a C₁-C₆ alkyl, alkenyl, alkynyl, or carbonyl, and wherein the donor solvent is N,N′-dimethylformamide, pyridine, imidazole, thiazole, oxazole, pyridazine, pyrimidine, pyrazine, triazine, pyrrolidine, pyrrolidone, isoxazole, isothiazole, or pyrazole.
 33. The method of claim 32, wherein the at least one metal is copper, nickel, iron, tin, indium, silver, gold or palladium or a combination thereof.
 34. The method of claim 32, wherein the donor solvent is N,N′-dimethylformamide, pyridine or imidazole.
 35. The method of claim 32, wherein the at least one metal is gold, silver or palladium.
 36. A method for separating a noble metal from a substrate, the method comprising: a) introducing the substrate to an organic solution comprising SOX¹X² and a donor solvent, b) reacting the noble metal with at least a portion of the SOX¹X² in the organic solution to form a reaction solution, wherein the reaction solution comprises a noble metal salt, and c) separating the reaction solution containing the noble metal from the substrate, wherein X¹ is Cl or Br and X² can be a halide, OR¹ or NR²R³, and R¹, R², and R³ are each independently a C₁-C₆ alkyl, alkenyl, alkynyl, or carbonyl, wherein the noble metal is gold, silver, or palladium or a combination thereof; and wherein the donor solvent is N,N′-dimethylformamide, pyridine, imidazole, thiazole, oxazole, pyridazine, pyrimidine, pyrazine, triazine, pyrrolidine, pyrrolidone, isoxazole, isothiazole, or pyrazole.
 37. The method of claim 36, wherein X¹ and X² are each independently Cl or Br.
 38. The method of claim 36, wherein the donor solvent is N,N′-dimethylformamide, pyridine or imidazole.
 39. The method of claim 36, wherein, before step a), the substrate is first treated with a solution comprising SOX¹X² without a donor solvent, then the substrate is separated from the non-donor solution and subjected to the remaining steps of the method. 40-47. (canceled) 